The invention pertains to a process for the sintering of a hollow cylinder of silicon dioxide soot, where the hollow cylinder is held in a vertical orientation by a holding device and is fed continuously through a heating zone where it is sintered. The invention also pertains to a device for holding hollow cylinders of silicon dioxide soot in a vertical orientation, with an elongated support body which extends from the outside into the bore of the hollow cylinder.
The production of preforms for optical fibers and preforms for furnace components for semiconductor technology frequently proceeds by way of an intermediate product, which is in the form of a hollow cylinder of silicon dioxide soot. These so-called "soot bodies" are mechanically weak and are therefore difficult to handle, especially when they are large and heavy. In addition, extreme requirements are imposed on the purity of the soot bodies for uses in optical communications technology and semiconductor technology, which means that the difficulty of handling them during the course of the various processing steps such heat treatments or gas treatments is further complicated by the restricted range of materials which can be used for the required processing aids.
When soot bodies of this type are treated in a treatment space such as the space inside a sintering furnace, one of the problems is how to hold them safely, since these soot bodies can withstand only a small amount of mechanical stress. In the process known from U.S. Pat. No. 4,251,251, the holder consists of a bracket shaped from platinum wires, from which the hollow cylinder of soot is suspended. The walls of the hollow cylinder have two horizontal through-holes passing through the upper end, through which the platinum wires are passed. For sintering, the soot bodies are fed continuously through a vertically oriented sintering furnace. In the furnace, a certain portion of the body becomes soft. This softening zone begins at the lower end of the soot body. As the soot body is lowered progressively through the sintering furnace, the softening zone migrates continuously upward along the soot body. Thus the weight of the sintered part of the soot body hanging from the softening zone increases continuously. Because of the low viscosity of the soot in the softening zone, the weight of the soot body itself is enough to stretch the body out. The process is therefore unsuitable for the sintering of large, heavy soot bodies.
U.S. Pat. No. 4,157,906 describes another device for holding blank in the form of a hollow cylinder of silicon dioxide soot in a vertical position during the cylinder-collapsing and fiber-drawing operations. A section of quartz tubing about 50 mm long is inserted into the bore passing through the blank. The outside diameter of the quartz tube is approximately the same as the inside diameter of the blank. The end of the tube which is to be inserted into the blank is provided with bump-like projections. To anchor the piece of tubing in the bore, these bumps are rotated about 90.degree. in the bore. The drive device used to lower the blank into the furnace grips a second piece of quartz glass tubing, which is set onto the end of the tube projecting from the bore.
U.S. Pat. No. 4,362,545 describes another device of the general type in question, in which an essentially sleeve-like handle of quartz glass is embedded in the bore of the soot body. At the end which engages with the soot body, the handle is provided with a circumferential, ring-shaped projection, which acts as a lock to prevent the handle from slipping out of the bore. The end of the sleeve projecting out of the end of the bore is designed as a cone, which is used to attach the handle to a device for suspending the soot body. The handle must be embedded while the soot body is being formed. For this purpose, the sleeve-like handle is pushed onto the deposition mandrel. The soot particles being deposited on the mandrel are also intended to cover simultaneously the end of the sleeve pushed onto the mandrel.
The known holding devices make it possible to hang hollow cylinders of silicon dioxide soot so that they can be sintered, for example, in a sintering furnace. Attaching the holders to the cylinders is a very complicated matter, however, and causes material to be rubbed off, which can lead to trouble. The known holding devices are suitable only for relatively small soot bodies. When the soot bodies are large and heavy, however, the mechanical stability of the hollow cylinder walls is no longer sufficient. There is therefore the danger that the holders will break away from the soot bodies.